1. Field of the Invention
The present invention is related to design characterization methods and circuits, and more particularly to integrated circuits having on-chip leakage current monitors, such as leakage current estimation or measurement circuits.
2. Description of Related Art
In very large-scale integrated circuits (VLSI) such as semiconductor memories and microprocessors, accurate leakage current measurement is typically not possible, since measurement of true leakage current requires that all of the devices under measurement be in an “off” state, which generally cannot be commanded at the same time across an entire integrated circuit die. Further, even if all leakage current paths have an “off” device (e.g., a CMOS circuit in a static state), both “off” devices and “on” devices vary in type and strength across the die. Leakage current across a die is difficult to model, and any model will deviate substantially from actual leakage current, as the combination of local variation with variation of the design values combines significantly, particularly in present-day low voltage technologies, in which the power supply voltages are decreasing to below one volt.
If a truly accurate leakage current monitor circuit were available, leakage current could be characterized across a circuit design on a die, individual device types could be characterized for leakage across voltage and temperature variations, and used to inform design decisions and monitor processes. A more accurate temperature monitoring function could also be provided, since device leakage can be used as an indicator of device temperature. Device leakage forms the basis for many temperature monitoring circuit designs, in which one or more reference devices conducts a temperature-dependent “off” state current, which is measured to generate a temperature analog. Further, leakage current values could be used as a control mechanism for maintaining energy use below a predetermined threshold, or for other purposes such as thermal control.
Present leakage current monitoring circuits typically measure a leakage of one or more devices to estimate total leakage of a device, but are not typically statistically representative of an integrated circuit design as a whole, with the exception of actual total device leakage current measurements, which can only provide measurement of the current consumed by an integrated circuit while the internal circuits are in a suspended state. Even in a suspended state, all devices are typically not in their “off” state, as noted above, and there is no ability to distinguish between leakage of different device types. Other test and measurement circuits may be provided to make leakage current measurements and/or model device current leakage, but also fail to provide a measure of true off-state leakage and fail to provide a model sufficiently accurate to the describe the actual integrated circuit.
Therefore, it would be desirable to provide methods, circuits and systems for leakage current monitoring that provide a measure of the true off-state leakage current for an integrated circuit design and provide an accurate, statistically descriptive measure of leakage current within the actual circuit being monitored. It would further be desirable to provide an accurate temperature monitor in an integrated circuit. It would be also be desirable to provide such a circuit that can provide an accurate leakage current value while the circuit being monitored is in an active operating state, and optionally control an environment and/or internal operation of the integrated circuit using leakage current monitoring results.